1. Field of the Invention
This invention concerns probes for coordinate measuring machines, particularly probes incorporating displacement tranducers of the electrooptic type in which optical gratings are utilized to generate an interference pattern (known as a Moire fringe) which shifts with relative movement between the two gratings in order to provide a probe which is capable of limited movement in a plane, which movement is detected by the probe transducer itself.
2. Description of the Prior Art
More fringe displacement tranducing arrangements as described in U.S. Pat. Nos. 2,886,717 and 2,886,718 and as described above have for various reasons enjoyed considerable success in the context of coordinate measuring machines, such as disclosed in U.S. Pat. Nos. 3,774,311; 3,757,423; and 3,403,448. These machines have usually taken the form of a probe shaft supported for linear movement along a first (Z) axis, the probe shaft support being supported for linear movement along a second (Y) axis orthogonal to the first axis on a carriage which is in turn supported for movement along a third (X) axis orthogonal to the first and second axes.
Movement along each axis is detected by means of a Moire fringe transducer, i.e., a scale grating is affixed to each of the supporting members aligned with movement therealong and an index price is carried by the member suported and positioned so as to produce the Moire fringe described.
For various reasons known to those skilled in this art, it is often desirable for the probe shaft to support a probe which is capable of limited detected movement in the plane orthogonal to its axis independently of the means of movement comprised by the supporting structure described above.
While it would be advantageous to detect such movement by a similar Moire fringe transducer means carried within the probe since the signals produced would be directly useable by existing system electronics, an arrangement has not heretofore been provided which was sufficiently compact to be carried within the confines of the probe.
For example, the two-axis transducer arrangement shown in U.S. Pat. No. 3,184,600 while producing a pulse type output upon movement of a probe in a plane utilizes a relatively bulky mechanical linkage to maintain proper orientation of the axes, which approach would not be feasible in this context. In addition, the effective control by this linkage in precisely maintaining the orientation of the grid and the index piece carried by the probe is difficult to achieve.
Also, the proper orientation and spacing of the index and scale gratings may be distributed upon bumping or dropping of the probe in this arrangement.
Finally, the bearing system for such arrangements must be quite friction-free since frictional drag produces slight structural deflections causing errors in measurement.
Accordingly, it is an object of the present invention to provide an arrangement for supporting the index and grid grating for movement in a plane which is frictionless and sufficiently compact for application in such probes and will precisely maintain the proper orientation and spacing therebetween notwithstanding the imposition of inadvertent excessive loadings on the probe tip tending to affect the grating alignments.